This invention relates to logic devices such as programmable logic devices (“PLDs”), and more particularly to the logic modules used in such devices.
Logic devices typically include many instances (replications) of basic circuitry called a logic module. Because this basic circuit unit is replicated so many times on a logic device, it is very important for it to be both powerful and efficient. By “powerful” it is meant that the logic module is capable of as many different, commonly needed tasks as is reasonably possible. By “efficient” it is meant that the logic module does not include more circuit elements than necessary, that it is not characterized by any more signal propagation delay than necessary, etc.
A typical logic module is capable of providing a primary output signal that is any logical function of a predetermined number of primary input signals to the logic module. For example, it is very common for a logic module to have four primary input signals. It is also frequently desirable for a logic module to be able to perform one digit or bit of binary addition (or subtraction) and one digit or bit of binary multiplication. (For ease of reference herein, “addition” will generally be understood to also include subtraction.) Parallel addition or multiplication of several digits or bits of binary data is typically what is desired, so several logic modules are typically involved. To help support such parallel arithmetic operations, carry connections may be provided between logic modules. In other words, in addition to its primary inputs and its primary output, a logic module may have a carry in input that comes substantially directly from another adjacent or nearby logic module, and a carry out output that goes substantially directly to yet another adjacent or nearby logic module. In the case of addition, for example, a logic module receives two addend signals via two of its primary inputs; it receives a carry in signal (from the logic module performing the next-less-significant digit position of the addition) via its carry in input; it produces a sum out signal via its primary output; and it outputs a carry out signal (for use by the logic module performing the next-more-significant digit position of the addition) via its carry out output.
Logic module circuitry is needed for efficiently augmenting the basic combinational logic capability of a logic module with arithmetic capability (e.g., the handling of a carry in input, the production of a carry out output, and the performance of one digit or bit of an arithmetic operation such as addition or multiplication).